The present invention relates to optical communications and, more particularly, to planar optical waveguide devices for use as components in optical circuits.
Waveguide devices of various types, including some, such as modulators, mode converters, and switches requiring an input of electrical energy for their operation, are needed to direct and control electromagnetic carrier waves in the optical region of the spectrum for conveying information. Waveguide devices of planar construction have been found advantageous for many applications because of their small size, durability, low cost and ease of fabrication. In such planar waveguide devices, the waveguide can be formed as a channel in a planar substrate of crystalline birefringent light propagating material by various methods to raise the index of refraction of a localized portion of the substrate. Commonly used procedures for raising the index of refraction of a light propagating material include ion exchange processes by which, for example, titanium is diffused at high temperature into the material or the material is immersed in a bath of melted benzoic acid. For waveguide devices requiring electrical energy, the energy is applied through electrodes, normally in the form of thin strips of metal deposited on, over or near the waveguide. One problem with the presence of metallic electrodes near the waveguide is that the guided waves are attenuated by the optical absorption of a portion of the energy of the waves in the vicinity of the electrodes.
In order to protect the waveguide from the attenuating influence of the electrodes, one practice has been to deposit over the surface of the substrate, as by a sputter deposition process, a thin dielectric film, for example, about 0.1 micron of silicon dioxide, having an index of refraction lower than that of the waveguide and then to deposit the metallic electrodes onto the dielectric film. The dielectric film must be of high optical quality, that is, it must be free of defects such as impurities or occlusions that define light scattering centers which contribute to the attenuation of guided waves. The dielectric layer must have an index of refraction equal to or less than that of the substrate, must also have a thickness sufficient to prevent the wave energy field from coming into contact with the electrodes, and an electrical conductivity sufficiently low, that is, a resistivity sufficiently high, to prevent current from passing into the substrate when electrical energy is applied at the electrodes.
It is difficult to produce dielectric films of the required quality and electrical conductivity to effectively isolate the waveguide from the metallic electrodes. A high degree of quality control is required to produce films free of defects. This requires providing an ultraclean environment, employing very pure materials and precisely controlling temperature, humidity and vacuum during the manufacturing process. Slight deviations from optimal conditions result in substantial reductions in the yield of high quality material.
Furthermore, even though crystalline birefringent light propagating material of high purity can be produced for use in waveguide devices, such material still contains trace amounts of impurities, notably iron, which can be photoionized and react with the light propagating through the waveguide. As a result, a space charge near the waveguide accumulates and perturbs the guided modes through the electro-optic effect. Such phenomena are usually classified as optical damage effects.
Additionally, waveguides formed with the foregoing processes tend to have asymmetric modal power distributions with respect to the axis of the waveguide. Due to this asymmetry, the coupling efficiency of light to and from these waveguides is greatly reduced.
Among the objects of the invention, therefore, is to protect optical waves in the waveguide of a planar waveguide device from attenuation by metallic electrodes by forming a buffer layer in the substrate of the planar waveguide device and forming the buffer layer by a process which can be precisely controlled and reliably produces a defect-free buffer layer.
Another object of the invention is to increase conductivity in the region of the substrate near the waveguide to reduce space charge accumulation and substantially reduce optical damage effects.
Other objects and further scope of applicability of the present invention will become apparent from the detailed description to follow taken in conjunction with the accompanying drawings in which like parts are designated by like reference numerals.